Modern telecommunication circuits, such as telephone systems, rely on multiplexing to pack more information onto a single wire or cable. Such systems typically employ time-division multiplexing which takes small time slices of each of many different signals and sequentially packs these time slices together to form a higher-rate multiplexed signal.
For example, modern telephone systems convert speech in a telephone signal into a digital data stream having 64,000 bits per second (64 kbps). Such data streams are known in the telecommunications industry as Digital Service, Level 0 (or DS-0). A simple multiplexer can take small time slices (or frames) of 24 different DS-0 data streams (from 24 phone lines) and combine these time slices sequentially into a higher rate data stream of 1,544,000 bits per second (1.544 Mbps), which is known as Digital Service, Level 1 (or DS-1). Note that 1.544 Mbps is slightly greater than 24 multiplied by 64 kbps, to accommodate the addition of synchronization or framing bits. A DS-1 signal is normally carried on a T-1 digital transmission link, which typically includes two pairs of twisted wires. One twisted wire pair carries a DS-1 signal in one direction and one twisted wire pair carries a DS-1 signal in the opposite direction.
In a similar fashion, multiple DS-1 signals are multiplexed together to form even higher rate signals. For example, 28 DS-1 signals can be multiplexed together to form a higher rate data stream of 44,736,000 bits per second (44.736 Mbps), which is known as Digital Service, Level 3 (or DS-3). Note that 44.736 Mbps is slightly greater than 28 multiplied by 1.544 Mbps, to accommodate the addition of framing bits. A DS-3 signal is carried on a T-3 digital transmission link, which may typically include a pair of copper coaxial cables, although fiber optic or RF transmission systems can be used as well. Since each DS-1 signal may carry 24 different telephone conversations, each DS-3 signal may contain 672 different telephone conversations.
Multiplexing devices for converting between DS-1 signals and DS-3 signals have been in use for some time now and are commonly referred to as M1-3 multiplexers. Unfortunately, most of the Me-3 multiplexers in use today are based on technology from the late 1970's. Further, the Me-3 multiplexers currently being marketed are not very different from those older Me-3 multiplexers still in use. Specifically, Me-3 devices are generally large in volume and weight. Telecommunication equipment is oftentimes mounted in vertical racks having a width of either 19 or 23 inches. Within these racks, a vertical space of 1.75 inches is typically provided in which to install a given piece of equipment. This space is known as a "rack unit" or (RU). Older M1-3 devices may have required up to 2 feet of vertical space on the rack, or 8 RUs. Modern M1-3 devices are typically at least 3 RUs tall. With the proliferation of increasingly sophisticated telecommunications equipment and the distribution of same to customers' premises (M1-3 devices may now be installed on-site at large corporations), it is desirable to significantly decrease the volume of space used by each device, such as an M1-3 device. Radically different designs may be required to achieve such a decrease in volume.
Another issue with M1-3 devices is their ability to perform self-tests and assist in testing of the telecommunications equipment to which it interfaces. As can be appreciated, when a device impacts as many telephone lines as an M1-3 device does, and with the increased reliance on telephone lines to transfer digital data between computers, the proper operation of the telecommunications equipment is of paramount importance. One form of network testing includes generating a signal including a pseudorandom bit sequence (PRBS) at one location in a telecommunications circuit, receiving the PRBS at another location in the circuit, and comparing the received signal to the expected signal to determine the accuracy with which the signal was propagated through the circuit. This accuracy is typically expressed in terms of a bit error rate (BER). Particular sections or components of a telecommunications circuit can be fault-isolated through a technique known as "loopback." A loopback is a temporary condition in which an outgoing signal is reflected back as an incoming signal to isolate one section of the telecommunications circuit so that more specific detection can be made of the malfunctioning equipment. The ability of current M1-3 devices to perform such network tests and loopbacks has been limited. Specifically, it is believed that current M1-3 devices cannot generate or detect a PRBS to test the network or any portion thereof. In addition, current M1-3 devices cannot create loopbacks (or detect loopback codes) to facilitate testing. In order to interface with the low speed network on one side of an M1-3 device, it is typically necessary to use 28 different network interface units (NIU), one for each T-1 line. These NIUs are able to detect loopback codes sent on the T-1 lines and perform the loopback function by routing the receive signal to the transmit signal path in response to the loopback codes. In addition, different types of NIUs are available for performing a similar function on the T-3 side of M1-3 devices. Thus, a total of 29 different NIUs may typically be used with an M1-3 device.
Because of the number of telephone calls which may be simultaneously routed through an M1-3 device, and because of the remote locations where M1-3 devices may be installed, it is desirable for M1-3 devices to have the functionality to remain operational even when certain internal components and/or external equipment have failed. For this reason, M1-3 devices have some redundant or spare components provided therein which may be automatically switched in to replace the failed components. Typically, the spare component is switched in for the failed component via electro-mechanical relays. Because of the mechanical aspects of relays, the transition may take as long as 5 milliseconds to complete. At DS-1 rates of 1.544 Mbps, this transition time may be tolerable, but at DS-3 rates of 44.736 Mbps this transition time will cause an unacceptable amount of errors and will create alarms undesirably. It would be preferable to have an M1-3 device which did not set off alarms when switching in/out DS-3 level equipment. Such a device would be said to have "hitless" transitions if no alarms were set off. Of course, even with hitless transitions, there would be some small number of errors and loss of data, but not a sufficient amount to set off alarms per the applicable regulatory specifications.
In the DS-1 portion of most M1-3 devices, there are a plurality of circuit cards to interface with the 28 T-1 lines of the low speed network communicating to the M1-3 device. Each of these circuit cards may include sufficient interface electronics for 4 of the T-1 lines, meaning that 7 circuit cards may be needed for the 28 T-1 lines. A redundant or spare circuit card may be provided with sufficient interface electronics to interface with 4 T-1 lines. Some M1-3 devices allow the spare card to be switched in to replace one of the afore-mentioned 7 cards if a failure is detected. If, however, there is a failure in one set of interface electronics on one card and in another set on another card this system will not provide sufficient redundancy to allow the M1-3 device to remain completely operational.
In addition, it is believed that current M1-3 devices do not internally provide for network redundancy without additional external equipment such as a network interface unit. Network redundancy allows the telecommunications system to continue to operate even when a T-3 communications link fails. To provide this redundancy, systems may provide two T-3 links on diverse routes. For example, one T-3 link may be via through-the-air RF transmission, while a second T-3 link may be via an underground copper coaxial cable. This route diversity decreases the likelihood of a simultaneous failure in both links. Current M1-3 devices must be attached to an external network interface unit in order to interface with or connect to more than one T-3 communications link.
Another issue with telecommunications equipment is the desired ability to have equipment which can be monitored and controlled externally via a computer network when desired. To the best of applicants' knowledge, the only prior and current M1-3 devices which can be externally monitored or controlled can be done so only via a dedicated computer or terminal.
It is against this background and the desire to solve the problems of the prior art that the present invention has been developed.